Conventional communication systems include a receiver for receiving and processing transmitted waveforms. One type of receiver is part of a "wireless cable" system that allows consumers to receive directly in their homes up to 150 television channels broadcast from a pair of powerful satellites. The receiver includes a small 18 inch satellite dish connected by a cable to a video processor unit. The satellite dish is aimed toward the satellites, and the video processor unit is connected to the user's television in a similar fashion as a conventional VCR.
On the transmission side, video and audio signals are digitally encoded into a data stream using a number of algorithms, including convolutional error correction algorithms. The encoded data stream is then compressed to reduce bandwidth requirements, modulated up to a high frequency Ku-band signal, transmitted to the satellite, and relayed from the satellite to the satellite dish located at the user's home. The satellite dish shifts the Ku-band signal down to an L-band signal which is transmitted through the cable to the video processor unit.
In the video processor unit, front-end circuitry receives the L-band signal and converts it into the original digital data stream of video/audio signals. The digital data stream is fed to video/audio decoder circuits that perform video/audio processing functions such as demultiplexing and decompression. A micro-controller controls the overall operation of the receiver, including the selection of receiver parameters, the setup and control of front-end circuit components, channel selection, viewer access to different programming packages, blocking certain channels, and many other functions.
In the above-described system, the receiver needs to lock onto a carrier signal within the frequency band of the digitally modulated L-Band signal before the signal can be converted into the original digital data stream of video/audio signals. The carrier signal is known to be within a predefined frequency band. However, due to transmission effects, the exact frequency and phase of the carrier signal is not known.
A traditional method of locking onto the carrier signal involves first setting a threshold value in the receiver based on an expected signal quality factor (SQF) value. The receiver then sweeps the entire frequency band measuring the signal quality. The receiver attempts to lock onto the carrier signal when the measured SQF exceeds the threshold value.
Under the above-described locking method, the threshold value cannot be set in the receiver with great accuracy primarily because signal to noise ratio for the carrier signal is not known in advance. If the threshold value is set too high, then the receiver may not lock onto the signal. If the threshold value is set too low, the receiver may incorrectly lock onto background noise or a signal based on a carrier harmonic.
Accordingly, there is a need for a method and device for quickly and accurately locking onto a carrier signal located within a particular frequency band.